(Provided by Peter Dmytrasz, Certified Arborist)
Mature tree pruning, when undertaken by the inexperienced, is one of the most dangerous and life-threatening activities a homeowner or property manager can undertake. Those foolhardy enough to try, then live to tell about it, can attest to the grotesque appearance and severe harm to the vitality or structural integrity of the tree that can be caused by the indiscriminate cutting of limbs and branches. Missing fingers hands or feet are not too pleasant to look at either. Professionals take years of combined study, training and apprenticeship before safely working in mature trees.
Many municipalities currently administer, or have the enabling legislation to institute, private tree bylaws. Proper tree pruning takes into consideration the physiology of tree growth, tree health and other influencing factors. Professionals in the private tree care industry, commencing with the local municipal forestry section, should be consulted for assessment of the proposed work.
Properly maintained trees greatly increase the value of any property. Reasons for considering the professional pruning of mature trees are varied. Some are essential to the structural integrity and health of the tree while others are purely cosmetic. Wound dressing (paint) is not recommended as it may interfere with natural wound closure or in many cases may actually accelerate decay.
Trees with co-dominant leaders or other tight main crotch angles with included bark between them tend to split easily, especially during wind, or ice storms. The weaker or the more laterally positioned limb should be removed, ideally when the tree is young.
Crown cleaning or removing undesirable, weak, dead, insect or disease infected limbs, suckers or watersprouts, mechanically damaged limbs, rubbing or crossover branches, and small girdling roots, those that have wrapped themselves around the main stem.
Crown restoration is required, for lightly storm damaged trees or trees previously pruned for crown reduction, to eliminate profuse shoot production at the previous terminal pruning cut.
Crown reduction may be required to reduce the spread or height of a tree, especially if there is interference with hydro wires or with buildings, existing or under construction.
Crown thinning, or the selective limb removal increases air movement and light penetration, for better foliar disease resistance and reduces the wind sail effect of dense tree crowns.
Crown raising, or the removal of lower branches, for safer use of the property by owners, guests and their vehicles, or for elevating crowns for vistas may be required.
Pruning can impact the size, quantity and quality of flowering and fruiting. Selective pruning generally delays flowering and encourages fewer but larger flowers and resultant fruit. Special effects and appearances of certain trees can be achieved through pruning by creating espalier, formal hedges, topiary, pollards or bonsai.
Pollarding or topping mature trees is not considered a good or proper arboricultural practice. Invigoration of trees with an extreme lack of vigour and doing very poorly may be shocked into improved growth by drastic pruning. This usually has the ‘kill or cure’ effect.
Nursery bought trees are often pruned for attractiveness at the time of sale, while trees grown in the wild are usually scraggly, requiring proper framework training in their new site. Trees with properly framed primary, or scaffold, branches develop into structurally strong specimens with long useful life spans and require minimal corrective pruning as they mature.
The pruning of small young trees, especially during the first 3 to 5 years in their permanent site, are generally within the capability of the average home owner or property manager. Angled cuts should be made with a sharp pair of pruning shears or a sharp pruning hand saw in the direction of and just above an outward pointing bud or branch union. Ideally the bud or branch should be pointing in the direction of desired growth. Stubs may potentially become diseased and should be avoided, while cutting too close may damage or weaken the branch.
Very small trees may not be tall enough for selecting the first permanent, primary scaffold branch. The entire crown may be composed of temporary branches that will eventually be removed.
Keep in mind that no more than 30 % of the tree should be removed in any one year. Dead, damaged and diseased branches, including roots if the tree is a bare root specimen prior to planting, should be removed.
Weak tight branch angles with included bark at the trunk union should be avoided. Prune away the weaker or potentially interfering branch. Strait sturdy tapered trunks with well-spaced lateral branches, both vertically and radially, are desired. Chose ones that are between one-third and one-half the trunk thickness.
Timing of pruning depends on the type of tree, the tree’s condition and the intended results of the pruning. Generally for healthy trees under normal conditions it is just before the period of rapid growth in the spring. Deciduous trees are generally best pruned during the dormant season when the leaves have fallen and the view of the branching structure is unobstructed. Early winter pruning, when the sap flow is reduced, is preferred for bleeders or trees that ooze sap profusely such as birches, elms and maples.
Disease control can be achieved in some cases by timing pruning appropriately. The pruning blades should be sterilized between cuts to ensure against the re-infection of healthy trees. Autumn is usually the time that wood decay pathogens are sporulating and major pruning activities should be avoided, especially for the larger or mature trees. Spore carrying beetles can spread vascular diseases like Dutch elm disease and Oak wilt. Prune susceptible trees when they are dormant and the adult beetles are not present. Trees with Fire blight, mainly fruit trees and mountain ash, are best pruned in dry weather to reduce the risk of spreading the disease with wet pruning shears. Trees with Black knot, mainly cherries, plums and other stone fruit trees, should be pruned in the dormant season prior to spring flush when the cankers sporulate.
Flowering trees can maximize their floral displays if pruned immediately after flowering. Next year’s flower buds have sufficient time to develop during the rest of the growing season. Trees reluctant to flower or fruit can be stimulated by digging a trench and severing the roots around half of the tree just inside the drip line, or outer extent of the crown, one year and the other half, the next year.
Coniferous trees are best pruned in late spring after the new growth has started to harden off, which is usually late May or June in southern Ontario. The exceptions are for large limb removal, best done in the dormant season and for pines, best done during the candle stage or immediately after the completion of the new shoots.
These tips are provided to make you aware of some of the complexities involved, when considering tree-pruning activities. There are many good books that illustrate various pruning techniques and specific considerations related to the type or even specie of plant to be pruned. When in doubt, always consult a specialist.